Spectroscopy has been a reliable, omnipresent, powerful tool for both qualitative and quantitative analysis. In its qualitative aspects a variety of spectroscopic techniques have been employed, alone or in combination, to identify materials or their components. In its quantitative aspects spectroscopic methods have been most commonly employed to determine such primary properties as concentration, or the amount of one or more components in any material. More recently spectroscopic measurements have been performed at multiple wavelengths with attendant multivariate analysis of transmittances at the measured wavelengths to calculate derived properties such as octane number or a physical property such as polymer melt index. This technique is very general, and merely posits a relationship between some totality of wavelength-transmittance measurements and the derived property, e.g., octane number, a relationship which is determined via appropriate analysis (partial least squares, principal component regression, multiple linear regression, etc.) of substrates with known values of octane number to afford a predictive equation for the domain established by the substrate sample class. The equation has the form, ##EQU1## where P is the property to be measured, T.sub.j are the transmittances of light at wavelength j.sub., n is the number of wavelengths at which measurements are made, and b.sub.j are the coefficients determined from multivariate analysis. Although this technique of multiple wavelength analysis is most often applied to what we refer to above as derived properties, it also can be used advantageously for measuring the concentration of a component, particularly where 1 or more interfering substances are present. By "interfering substances" we mean substances which have significant absorbtion at those wavelengths commonly used to measure the concentrate of the analyte in question.
To date the foregoing technique using measurements at multiple wavelengths in conjunction with a predictive equation employ computational methods subsequent to, and independent of, the spectroscopic measurements themselves. In addition, measurements at multiple wavelengths generally employ means to separate a broadband energy source into its component wavelengths, as by use of a diffraction grating or prism mounted on a rotatable platform which disperses the electromagnetic radiation into discrete wavelengths, with a different wavelength falling on the sample as the rotation angle changes. This requires cumbersome and delicate mechanisms which substantially increase instrument cost. An alternative is to have nondispersed light pass through the sample, then disperse the transmitted light into discrete wavelengths, each of which falls on a separate diode in a diode array. This approach, however elegant, also results in increased instrumental complexity and increased instrument cost.
The broad purpose of our invention is to perform accurate property measurements using spectroscopic measurements at multiple wavelengths, but using 1) nondispersed radiation with but a single detector--or in some cases two detectors--sensing all the requisite wavelengths simultaneously, and 2) optical calculation of the property but without needing the usual computational accoutrements. This leads to substantial reduction in instrument costs through simplified design and a reduction in components, with concomitant increased ease of use, and the elimination of the need for ancillary computational means.
Our invention is applicable to spectroscopy generally, whether based on the intensity of electromagnetic radiation transmitted, reflected, emitted, or scattered. Because we employ optical methods our invention is directed to spectroscopy where the electromagnetic radiation has a wavelength in the range from about 150 nm to about 15,000 nm. Aside from this restriction it will be seen that our invention is quite broad and quite general in its application to spectroscopy.
It needs to be mentioned that Myrick et al., Anal. Chem., 1998, 70, 73-82 have recognized that it should be possible to process multiwavelength spectra by passing the multiwavelength light, without dispersion, through a filtering mask with wavelength-dependent transmittance factors onto a common detector. Myrick's implementation, however, is substantially different from ours. In particular, the authors' mask represents a regression vector as a continuous function over the entire wavelength range, i.e., it is a homogeneous filter; the authors note the difficulty of fabricating such a mask or filter. In our invention the mask represents a discontinuous function, with the filter being everywhere opaque except at a relatively small number of wavelengths where measurements are made. As we note within, such filters are easily fabricated in several ways; there is no problem with filter fabrication as there is with the implementation of Myrick et al. Our filter can be viewed as a massively parallel filter, or mosaic filter, in contrast to the authors' implementation.